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Old 29-07-03, 08:09 PM
Rosemary Rosemary is offline
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Join Date: Jun 2003
Location: Seattle, WA U.S.A.
Posts: 65
Default salts

I would guess that the stuff is sodium chloride or sodium sulfate. But the description of it being very light might indicate sodium bicarbonate or sodium carbonate which would occur if the pigment had a residual amount of excess sodium ions present to maintain its basic nature to be more stable.

Sodium chloride has colorless cubic crystal shapes, sodium sulfate heptahydrate has white rhomboid or tetragonal crystals, sodium alluminum silicate has colorless hexagonal crystals, sodium aluminum sulfate has colorless cubic octagonal crystals. sodium carbonate heptahydrate has white rhomboid bipyramidal efflorescence.

So if you can look at the crystals with a magnifier, you might get an idea which of the above is contaminating the pigment, unless it is a mixture which will affect the crystal shapes.

As the stuff is water soluble, successive suspension of the pigment in water, letting it settle and decanting the water several times should remove the bulk of the salt. But I have had the problem of wet ultramarine blue solidifying in the jar.

I have not had salt efflorescence with ultramarine blue, but I store it dry and mix with water just before using. I am using ultramarine blue dark from Old Holland.